In the oil and gas industry, bores are drilled to access sub-surface hydrocarbon-bearing formations. Conventional drilling involves imparting rotation to a drill string at surface, which rotation is transferred to a drill bit mounted on a bottom hole assembly (BHA) at the distal end of the string. However, in directional drilling a downhole drilling motor may be used to impart rotation to the drill bit. In such situations it tends to be more difficult to advance the non-rotating drill string through the drilled bore than is the case when the entire length of drill string is rotating. The applicant supplies an apparatus, under the AG-itator trade mark, which may be utilised to induce vibration or movement to parts of a drill string, and which apparatus has been found to increase the rate of progress (ROP) of drill bits during some directional drilling operations. Features of this apparatus and other tools capable of inducing vibration or agitation may be found in applicant's U.S. Pat. Nos. 6,279,670, 6,508,317 and 6,439,318 the disclosures of which are incorporated herein by reference.
Applicant's AG-itator apparatus includes a Moineau principle positive displacement motor (PDM). As drilling fluid pumped through the drill string drives the motor, the motor stator drives a valve arrangement to vary the flow of fluid through the lower end of the drill string. The varying or pulsing fluid flow acts on a shock-sub which tends to extend and retract in response to the pressure variations in the fluid in the string resulting from the operation of the valve.